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>> Tuesday, October 28, 2008

On the demand side, agricultural users are becoming more numerous and increasingly must compete for water with industrial and residential users—a conflict already playing out in urbanizing regions from coastal China to the western United States, and especially California, whose farms suck up four-fifths of the state's water. On the production side, the picture is more complicated, because not all agricultural water is created equal. Farms can be supplied by two kinds of water: water that comes directly in the form of rain, and water that is stored in rivers, lakes, glaciers, reservoirs, and underground aquifers. The difference between these two forms is critical. Rain-fed, or "green," water, in hydro-logical parlance, is what is known as a free good—it falls from the sky and doesn't need expensive reservoirs, dams, irrigation ditches, or wells; rain-fed crops also tend to be more water-efficient than those fed from stored water, because stored water—also known as "blue" water—has to be transported, when a huge amount is lost via leakage and evaporation. (Moreover, groundwater often contains traces of dissolved minerals, including salt; when such water is used for irrigation, these minerals accumulate in farm soils and can destroy yields.) Because of these differences, green-water agriculture can generate up to five times the calories for the same ton of water as blue-water agriculture can.

The downside to green water is that it's finite: once you've put the last rain-fed acre into production, you can't expand your green-water resource. Instead you must supplement your rain-fed capacity with blue water, which is what the great push for irrigation systems in the last century was all about. And while tapping our blue-water resources was a big reason we could feed so many additional people, it's also why we're in such dire straits now. Whereas green-water supplies are effectively self-regulating—once you use up all the rainfall, you can't use any more—blue-water resources can be exploited faster than they can be replenished. Rivers can be pumped so extensively they no longer flow year-round, or at all. Aquifers can be drawn down faster than they can be recharged by rainfall. (And emptied, they remain that way.) In India, a country whose food self-sufficiency was essentially paid for with overdrawn blue water, aquifers have been so heavily tapped that water tables are falling by up to twenty feet a year. In North Africa, water is being withdrawn from aquifers as much as five times faster than it can recharge, forcing farmers to drill their irrigation wells to depths of nearly a mile. Even in the rain-rich United States, the huge Ogallala Aquifer, which supplies one in five irrigated acres nation-wide, is being overdrawn at a rate of 170 million tons (3.1 trillion gallons) a year and is gradually forcing many farmers to either shift to new "dry land" crops or abandon agriculture altogther.

Perhaps the most serious overuse occurs in eastern China. In the so-called 3-H region, a massive area covering the Huang, Hai, and Huai river basins that contains 40 percent of the nation's population, half its grain production, but just a tenth of its water resources, water use now exceeds the sustainable flow by more than six hundred million tons a year, according to a 2001 report by the World Bank. Overdrafting is so severe that water tables have fallen by up to three hundred feet, ground levels are subsiding (Beijing's elevation has actually dropped by several meters), and in coastal areas, freshwater wells are now sucking in seawater. All told, reports one researcher, China is now thought to be feeding perhaps two hundred million people—around a sixth of its population—with water withdrawals that cannot be sustained.